The jihadists stoning women to death in Mali and taking hostages in Algeria are harbingers of much worse to come. Osama bin Laden may be dead, but al-Qaida in Africa now threatens an area twice the size of Germany.

Mali is just one country in the Sahel, a million square miles of arid and semi-arid countryside stretching from the Atlantic to the Red Sea. The region has always been subject to episodes of starvation and brutal tribal conflicts, and things are now deteriorating further.

Many of the region's problems today can be traced back 20 years, to a decision by the United Nations and other international organizations to shift their focus away from family planning, which we know to be an extremely effective way to empower women and stretch scarce resources.

Since then, a variety of agencies have proclaimed their commitment to women's rights, but nothing concrete has been done to change the atrocious treatment of women in places like the Sahel. Girls as young as 10 are married to 40-year-old polygamous men. The obscenity of female genital mutilation is widespread. Women do much of the work, often walking 10 miles a day to collect water. Most are illiterate: In Niger, for example, only one girl in 1,000 completes secondary school.

Twenty years ago in Mali there were 6 million people. Today, there are 16 million, and that number is expected to grow to more than 35 million by 2050. Could that be one reason al-Qaida now controls half the country? The 9/11 Commission investigating the al-Qaida attacks on America 12 years ago called a rapidly increasing population of young men without any reasonable expectation of employment "a sure prescription for social turbulence."

The population of the Sahel as a whole will have increased more than tenfold in a single century, from 31 million people in 1950 to 340 million by 2050. Even with good governance (which the Sahel lacks), it is impossible to provide educational and employment opportunities for populations growing at this rate. As the 9/11 Commission report foresaw, social turbulence in the Sahel is turning into whirlwind of violence.

On top of this already daunting catalog of problems, climatologists at the Lawrence Berkeley National Laboratory project that temperatures in the Sahel will rise by an additional 7 to 10 degrees Fahrenheit by 2050. Crops will wither and livestock will die even as the population continues its rise. The crisis unfolding in the Sahel could become a cataclysm affecting 200 million people.

Without radical new policies, we can be certain that there will be more conflicts, more failed states and more easy pickings for Islamic fundamentalists. The only genuine path to peace in the Sahel is by investing in women. It will be a long, difficult process, but it is not impossible if we start today and on a realistic scale. As a result of the failure to invest in family planning for 20 years, only 8 percent of women in Mali use contraception.

It doesn't have to be that way. The Bixby Center for Population, Health and Sustainability at UC Berkeley has been working with Nigerian colleagues in a group of villages near the northern border with Niger. We found that the average girl there was married at 14 and had given birth to two children by age 18. Almost none entered secondary school, much less completed it. In a tiny but highly successful project, working closely with communities, we were able to keep 205 out of 230 girls in secondary school. Only three of those who remained in school married before graduating, and only one had a child. The cost of the program was less than $40 per girl annually.

It will take 20 years and billions of dollars to bring projects such as this to scale, but there is no other plausible way to bring stability to the region.

The U.S. military has spent $620 million training local armies in counter-terrorism tactics in the Sahel. But that money hasn't bought the loyalty of testosterone-filled, volatile young men. The $620 million is actually proving counterproductive, as some of those who were trained used their new skills to overthrow their own government, while others built links with the hostage-takers in Algeria. The $620 million would have been better spent helping to give women the autonomy to decide when to marry and when to have children.

As a nation we spend almost $2 billion a day on defense. According to economist Joseph Stiglitz, the Afghan and Iraq wars will cost us a mind-boggling $3 trillion. And both wars have so far failed to create stable countries. One reason may be that both Afghanistan and Iraq are high-fertility, patriarchal societies, and our intervention did almost nothing to help women.

It's not that we don't need an army, and boots on the ground may sometimes be necessary. But the war or terror will continue to fail unless we convert a few days' military expenditure into investing in girls' education and family planning in the Sahel and elsewhere.

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Malcolm Potts, an obstetrician, is a reproductive scientist and professor of public health at UC Berkeley. In September 2012, he hosted a conference with colleagues from the region examining population and other issues in the Sahel. He wrote this for the Los Angeles Times.